1. Field of the Invention
The invention relates to a field device for process instrumentation and, more particularly, to a measuring transducer, comprising an analogue output, to which a two-wire line is connectable for transmitting an analogue output signal.
2. Description of the Related Art
Within the field of automation technology, field devices are frequently used for process instrumentation, which are used for instance to detect and/or influence process variables. The field devices are connected to one another by an automation network for the purpose of exchanging data. Field devices, which detect a physical or chemical variable as a process variable, are frequently referred to as measuring transducers, because they convert a relevant variable into a measured value and output this measured valve to a superior control station for instance or as an actual value to a controller for further processing. Examples of measuring transducers of this type are measuring transducers for fill level, mass flow rate, pressure, temperature, pH value, conductivity etc.
EP 1 192 614 B1 discloses a measuring transducer which converts a physical or chemical measured variable into an analogue output signal which corresponds to the measured value and which can be transmitted on a two-wire line. To this end, provision is made for a suitable sensor for the physical or chemical measured variable, an analogue/digital converter arranged downstream thereof, a computing unit arranged downstream thereof and an output circuit which is controllable by the computing unit and connectable to the two-wire line. Here, the sensor converts the measured variable into a sensor signal, which is digitalized in the analogue/digital converter and prepared in the computing unit to form a target value with which the analogue output signal is adjusted to the two-wire line by a control facility in the output circuit. The digital target value as a current default value is initially transformed into an analogue current target value in the output circuit using a digital/analogue converter and is converted into an analogue loop current by a subsequent control circuit. The actual value of the loop current is detected using a measuring element for the current/voltage conversion. Low-resistance current measuring resistors, such as shunts, can be used as simple measuring elements. The voltage signal thus generated, which is proportional to the actual value of the loop current, is fed to a comparison facility for forming a control deviation from the analogue current target value and the actual value of the loop current. With the aid of the control deviation, a controller, which is realized more clearly as an analogue controller with an integrated operational amplifier, in order to control the control deviation, determines an actuating signal for an actuating element, which is formed by a transistor circuit for adjusting the loop current. The measured analogue actual value of the loop current is simultaneously digitalized with the aid of an analogue/digital converter and fed to the computing unit as a digital actual value. The computing unit is therefore able to determine deviations between the digital actual value and the digital target value and to react thereupon as a result of a corresponding tracking of the target value. If impermissibly large deviations occur, this is communicated to the user of the field device. This can occur over a data interface and/or a display device for transmitting and/or displaying an item of information relating to the determined deviation and thus the measuring errors. The data interface is an integral part of the output circuit and enables data to be communicated by way of the two-wire line in accordance with the Highway Addressable Remote Transducer (HART) protocol.